Bacteria regulate internal pH. 1 5. Oxygen and Other Electron Acceptors § Many microorganisms can grow in the presence of molecular oxygen (O2). § Many organisms are killed instantly by O2. § Some even use oxygen as a terminal electron acceptor (TEA) in the electron transport chain. 2 Oxygen Has Benefits and Risks § Oxygen is a benefit to aerobes, organisms that can use it as a TEA to extract energy from nutrients. § Oxygen is toxic to all cells that do not have enzymes capable of efficiently destroying the reactive oxygen species (ROS)—for example, anaerobes. § For example, catalase enzyme. § Oxygen rich atmosphere is lethal to many organisms!!! 3 Aerobes, Anaerobes, and More § Strict aerobes can only grow in oxygen. § Microaerophiles grow only at lower O2 levels. § Strict anaerobes die in the least bit of oxygen. § Aerotolerant anaerobes grow in oxygen while retaining a fermentation-based metabolism. § Facultative anaerobes can live with or without oxygen. - They possess both the ability for fermentative metabolism and respiration (anaerobic and aerobic). 4 Culturing Anaerobes in the Lab § Three oxygen-removing techniques are used today: 1. Special reducing agents (thioglycolate) or enzyme systems (Oxyrase) can be added to ordinary liquid media 2. An anaerobe jar - O2 is removed by a reaction catalyzed by palladium 3. An anaerobic chamber with glove ports - O2 is removed by vacuum and replaced with N2 and CO2 5 5.5 Nutrient Deprivation and Starvation § Starvation is a stress that can elicit a “starvation response” in many microbes. • Enzymes are produced to increase the efficiency of nutrient gathering and to protect cell macromolecules from damage. § The response is usually triggered by the accumulation of small signal molecules such as cAMP or guanosine tetraphosphate, which globally transform gene expression. • These highly soluble, small molecules can quickly diffuse throughout the cell, promoting a fast response. 6 6 Microbes Encounter Multiple Stresses in Real Life § Bacterial stress responses have traditionally been studied in terms of individual stresses. § In the world outside of the laboratory, environmental situations can be quite complex, involving multiple, not just single, stresses. • For example, an organism could simultaneously undergo carbon starvation in a high-salt, low-pH environment. 7 7 Physical Agents That Kill Microbes § High temperature • Moist heat is more effective than dry heat. • Boiling water (100°C) kills most cells. • Killing spores and thermophiles usually requires a combination of high pressure and temperature. • At high pressure, the boiling point of water rises to a temperature rarely experienced by microbes. – Even endospores quickly die under these conditions. § Chemical Agents § Ethanol or Iodine or Chlorine (bleach) 8

Oxygen and Other Electron Acceptors

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